The present invention relates to a moulding material, particularly but not exclusively to a moulding material suitable for manufacturing lightweight articles which have a smooth surface finish and good mechanical properties.
Historically, moulded articles or mouldings were formed from resin either alone or reinforced with a fibrous reinforcement material. Although the products thus formed were satisfactory, it was difficult to guarantee the quality of the product due to the difficulty in controlling the ratio of the resin to the reinforcement fibres. The process was therefore refined such that the supplier of the resin provided the producer of the moulded article with preformed pre-impregnated reinforcement material known as a ‘prepreg’. This moulding material allows the moulder to produce moulded articles from the prepreg material at a consistent quality. This prepreg material also allows combined layers of reinforcement fibres and resin material to be laid up at once.
Prepregs are processed by applying heat together with pressure and/or vacuum to cure the resin. The viscosity of a prepreg resin system is strongly affected by temperature. On heating the prepreg material, the resin viscosity drops dramatically, allowing it to flow around fibres and giving the material the necessary flexibility to conform to mould shapes. However, as the prepreg is heated beyond a certain point (activation temperature) the catalysts within it begin to react and the cross-linking reaction of the resin molecules accelerates. The progressive polymerisation increases the viscosity of the resin in the prepreg until it has passed a point where it will not flow at all (‘no flow point’). The reaction then proceeds to full cure. Depending on the resin chemistry the curing process can take place at different temperatures, typically temperatures in excess of 40° C.
Within this application, if reference is made to the flow properties of any of the resin materials herein described and more in particular the viscosity of a resin material, it is referred to the flow properties of the resin during processing up to the point in time when the resin reaches its no flow point.
For some applications in for example the automobile and aerospace industry, it is desirable to produce articles (such as engine hoods, body panels, spoilers, etc.), which not only have a consistent quality but which are also light weight, and have excellent mechanical properties and a smooth surface finish.
A common moulding material for these applications consists of a sheet moulding compound (SMC). This material is essentially a prepreg comprising fibrous reinforcement material layers which are preimpregnated with a resin system. The resin system comprises a resin paste intermixed with a light weight filler to lower the density of the material. The addition of the filler also reduces the cost of the material. The SMC material is formed by applying fibrous reinforcement material layers onto a resin layer which comprises the resin system and filler to form a laminate. The laminate is pressed between impregnation rollers to drive the resin system into the reinforcement layers to form the prepreg SMC material.
The SMC material can be readily processed by applying pressure and/or vacuum to harden (cure) the material whilst the material is at a suitable processing temperature. One of the main problems associated with this moulded material is that the surface properties of this material are not ideal. During the processing of the moulding material whereby the laminate is heated, the viscosity of the resin paste is lowered. This causes the hollow filler, which has been dispersed in the resin paste and which has a lower density than the resin, to move upward and concentrate on the surface of the moulding material. After curing of the material, the surface of the composite material is not smooth and comprises deformations which can vary from small pin-holes to cracks. These pin-holes and cracks can develop into blisters during the life of the material. To effectively apply the cured material as an external surface, the surface is repaired by filling and fairing. This makes the production of mouldings from this material generally labourious, inefficient and expensive.
We have discovered that an alternative prepreg moulding material can overcome or at least mitigate some of the aforementioned problems. An embodiment of this alternative moulding material comprises a central layer which is sandwiched between two barrier layers. The central layer comprises a resin paste intermixed with a hollow filler similar to the SMC material as described above. A surface layer is provided on each barrier layer which is adapted to form a smooth external surface after processing of this material. The surface layer comprises a resin in which reinforcing fibres are arranged. Again, in the production of this alternative moulding material, the laminated layers are pressed and wound between impregnation rollers to drive the resin into the fibrous layers to form a prepreg sheet moulding compound (SMC).
An advantage of this material over the SMC material as previously described is that the barrier layer prevents the filler from emerging to the external surface. To that effect the barrier layer comprises a fibrous material which has a cell or pore structure which is sufficiently small to prevent the hollow filler from passing through the barrier layer. This material produces a surface which has reasonably smooth properties.
However, after the moulding material has cured, the reinforcement fibres, which are present in the surface layer, can be observed through the surface layer due to shrinkage of the resin. Further, reinforcement fibres in the surfacing layer which are not sufficiently embedded in resin, result in pin-holes on the surface. This prevents the material from being readily applied or coated without carrying out surface treatment operations such as fairing, filling or coating. A further disadvantage of this material, which we noted during the application testing of the material, is that, due to the high resin loading of the material, the prepreg material is difficult to apply in complex moulds since it is quite stiff and relatively inflexible (low drape).
Another factor which we encountered with this alternative material is that this material has little porosity through its thickness, so that substantial volumes of laminar gases such as air are trapped inside the laminate and on the surface of the mould during processing of the material, which results in voids in the cured laminate. Such voids result in the laminate having reduced mechanical properties which can lead to premature failure of the composite material. The gases which are trapped between the mould surface and the external surface of the material, result in surface imperfections such as pin-holes and cracks similar to the surface imperfections as discussed above.
It is therefore desirable to provide an improved moulding material and a method of forming said improved moulding material which allows more efficient fabrication of light weight moulded articles with enhanced mechanical properties and enhanced surface properties thereby addressing the above described problems and/or which offers improvements generally.